The marking of articles with bar codes for identification and sorting is well known. The conventional black on white bar code is useful only on areas of white or other light colored background on articles. Fluorescent bar codes can be applied anywhere on an article. Bar codes that are activated by ultraviolet light and fluoresce in the visible range have the disadvantage of interference from the background fluorescence of the article. Providing bar codes that fluoresce in the far red and near infrared substantially reduces the problem of background fluorescence interference.
A typical application of infrared fluorescing bar codes is for ZIP codes on postal material for sorting (Dolash, T. M., Andrus, P. G., and Stockum, L. A., "Novel Bar Coding System for Nonletter Mail", presented at Third Advanced Technology Conference sponsored by the U.S. Postal Service, Washington, D.C., May 3-5, 1988). Typically, the bar codes are printed with an ink-jet printer. The light source is a helium-neon laser and the detector includes a filter that blocks the activating light but passes the resulting fluorescent light from the bar codes. Typically, the ink is pigment-free and contains a fluorescent dye that absorbs the laser light at about 633 nanometers and produces a strong fluorescent signal at about 750 nanometers. The transparent blue bar codes do not obscure the underlying printed material. However, the reflectivity of the background can adversely affect the fluorescent signal and prevent accurate decoding.
A solution to this problem is provided in the copending U.S. patent application of Thomas M. Dolash, Paul G. Andrus, and Mark E. O'Loughlin, Ser. No. 392,171, filed Aug. 10, 1989, Background Compensating Bar Code Reader (assigned to Battelle Memorial Institute, the assignee of the present invention), now U.S. Pat. No. 4,983,817, issued Jan. 1, 1991. Separate detectors are provided to measure simultaneously the background reflectance and the fluorescence. The reflectance signal is used to compensate the fluorescent signal with suitable electronic circuitry to provide a background-compensated signal that can be used with conventional bar code readers to provide accurate decoding of the bar code.
Typically, the ink used in the above bar code reader contains an infrared fluorescing dye that absorbs in the red to allow use of the helium-neon laser at 633 nanometers. Typically the bar code is blue, enabling postal workers to see that a bar code has been printed, but it is kept light enough that it does not obscure names and addresses under it.
For some applications, such as in processing bank checks, involving document sorting equipment, the ink is formulated with a dye that fluoresces in the red and infrared and is also visible for human reading of the markings. U.S. Pat. No. 4,540,595 (Acitelli, et al) discloses a dye that fluoresces in the portion of the spectrum comprising wavelengths between 650 and 800 nanometers, in response to activating light in the range of 550 to 700 nanometers. Dyes that fluoresce in the range of 650 to 800 nanometers tend to absorb light in the visible spectrum and thus are useful for making human readable markings. The preferred human readable dye is a phenoxazine dye [3,7-bis(diethylamino) phenoxazonium nitrate] that in the preferred concentration of 0.09 to 0.12 percent has a visible light blue color. For use over light blue bank checks, the dye concentration is increased above the preferred range to 0.35 percent to make the markings more visible to the human eye.
Also in the prior art is another class of infrared fluorescent materials that can be activated in the infrared. These materials are inorganic compounds typified by the rare earths such as neodymium. U.S. Pat. No. 4,202,491 (Suzuki) discloses a data card wherein data are recorded with a powdery, inorganic infrared fluorescent material containing rare earths such as neodymium (Nd), ytterbium (Yb), and erbium (Er) that are activated in the infrared.
Fluorescent material containing Nd absorbs near 800 nanometers and can be activated with a gallium aluminum arsenide laser diode emitting at 800 nanometers. The maximum fluorescence is at 1050 nanometers. Fluorescent material containing Nd and Yb absorb near 800 nanometers and the fluorescence maximum is at 980 nanometers. Fluorescent material containing Nd, Yb, and Er absorb near 800 nanometers and the maximum fluorescence is at 1540 nanometers. Fluorescent material with only Yb and Er has a fluorescence maximum at 1540 nanometers but absorbs near 970 nanometers, necessitating a laser diode of gallium arsenide (silicon-doped) with an emission at 940 nanometers.
Typically, these inorganic materials are finely pulverized with a ball mill to an average grain diameter of 3 micrometers for use in printing inks. The inks dry to a white powder which is difficult to see on a white background surface but which would be visible on any colored background surface. Thus, to provide secrecy, the powdery material can be covered with a film that is opaque to wavelengths in the visible spectrum. Such a covering film also protects the powdery deposit from exfoliation in use. It is known that as the powders are made smaller in size, the fluorescent efficiency decreases and thicker films are required. Inks prepared from insoluble inorganic materials are generally not suitable for use in ink jet printers that are susceptible to clogging and those that produce thin films.
There are many applications in which it is desirable that the infrared fluorescing bar code be invisible to the unaided eye (i.e. no significant absorption in the visible spectrum from about 400 nanometers to about 700 nanometers). Such applications might include security needs or avoiding interference with the aesthetic properties of the package.
The present invention is based on use of infrared-fluorescing organic compounds such as laser dyes that are soluble in the ink vehicle. Specifically, the organic dyes of this invention can be activated in the infrared and fluoresce at longer wavelengths in the infrared. This class of materials is particularly useful for making inks for use in ink jet printers of bar codes. They can be invisible to the unaided eye over backgrounds of white or colored material, and when used in clear ink vehicles, the bar codes do not obscure the underlying background.
The present invention comprises a method for providing intelligible markings that are virtually invisible to the unaided eye on the surface of an article, comprising: applying, onto the surface, a marking medium comprising a compatible liquid or viscous substance containing an organic dye that is poorly absorptive of radiation in the visible range of about 400 to 700 nanometers, is highly absorptive of radiation in the near infrared range of at least about 750 nanometers in wavelength, and fluoresces in response to radiation excitation in the said near infrared range to produce fluorescent radiation of wavelengths longer than the wavelength of the excitation. Typically the dye is highly absorptive of radiation in the range of about 750 to 900 nanometers and the fluorescent radiation is produced principally in the range of about 800 to 1100 nanometers.
Typically the markings are applied by a jet printing process, and the dye comprises an organic laser dye such as IR-125, which typically is present in a concentration of about 0.005 to 0.05 percent by weight of the medium.
Other useful laser dyes may comprise DTTCI, DNTTCI, HDITCI, DDTTCI, IR-140, DDCI-4, or IR-132.